1. Field of the Invention
The invention relates to a process for the stabilization of cement stone which is formed from aluminous cementing material by setting with water against heat and humidity.
2. Descripton of the Prior Art
Contrary to portland cements formed with calcium silicates, alumina cements are principally composed of calcium aluminates mixed with small amounts of silicates, silicoaluminates, ferrites and in some cases titanates.
Alumina cements and other binding agents rich in aluminates are distinguished by fast hardening and fast attainment of stabilities which are far superior to those of portland cements. Hydrated alumina cements have proved to be extremely resistant to water attack and other chemical effects. Mortars, concretes and others, e.g. fibrous materials formed on the basis of aluminous binding agents, undergo a clear destruction after hydration at low temperatures if they are exposed to temperatures of higher than approximately 23.degree. C. This decay becomes evident by losses of strength which resulted therein, for example, that in some countries such as, for instance, the Federal Republic of Germany, alumina cement is no longer permitted for supporting structural parts.
It is well known to those with experience in the field that the heaviest loss of strength is to be expected when the structural parts are at elevated temperatures in air having high humidity. Under water, the decrease in strength is less strongly marked. According to current opinion, the following process is involved: The minerals CAH.sub.10 and C.sub.2 AH.sub.8 forming mostly during the normal hydration of alumina cement at temperatures below 23.degree. change to C.sub.3 AH.sub.6,AH.sub.3 and water at temperatures of above about 23.degree. C (in the formulas C=CaO,A=Al.sub.2 O.sub.3,H=H.sub.2 O). This reaction proceeds during substantial volume decrease of the solid phase, i.e., the cement stone becomes porous during this transformation. In view of the very rapidly proceeding exothermic hydration, large amounts of heat are set free which, in case of larger construction parts, can frequently not be removed quickly enough and thus set in motion the described transformation of CAH.sub.10 and C.sub.2 AH.sub.8.
According to the prior art it had been attempted to avoid the described drawbacks by decrease of th water-concrete proportion. By this method there is added to the concrete only an amount of water which is not adequate for complete hydration. However, this procedure leads to mortars which can only be processed with difficulty in view of their low plasticity.
It has also been attempted to prevent formation of the denser tricalcium aluminate hydrates by the addition of calcium carbonate, magnesium carbonate or calcium borates. To date no known measures have succeeded in avoiding the decrease in strength to a sufficient degree.